Despite advances in technology providing more fuel-efficient power generation for vehicles, efforts continue to strive for a more efficient vehicle overall. A large factor in vehicle efficiency lies in the aerodynamics of the vehicle. While the design of smaller road-going passenger vehicles adapts through continuous design revisions between model years, the road-going truck market, particularly the long-haul or Class 8 segment of the market has not been able to adapt as quickly. Also referred to as a “semi-truck” or “semi,” long-haul trucks transport mass quantities of goods through the use of trailers sometimes in excess of 50 feet in length and 60,000 pounds of payload capacity. The modern semi-truck trailer has undergone little design improvement for aerodynamic efficiency over several decades. Furthermore, the average fuel economy of a road-going semi-truck towing a loaded trailer is only 7.2 miles per gallon (Davis, Stacy C. 2014 Vehicle Technologies Report. Oakridge, Tenn.: U.S. Dept. of Energy, 2014. ORNL/TM-2015/85). There are currently over 5.6 million semi-trailers registered for use in the United States alone. The lifespan of an average semi-trailer typically spans 12-15 years, as such the immediate redesign of the standard semi-trailer will do little to improve overall efficiency in the near-term. As a result, there is a need for a near-term solution that improves aerodynamic efficiency of semi-trailers in a cost-efficient manner.
Aerodynamic drag is a primary contributing factor to fuel consumption when operating a road-going truck and trailer at highway speeds. Friction drag and pressure drag are two variables surrounding aerodynamic drag. Friction drag surrounds the interaction of the ambient air and the surface of the trailer as it moves through it. However, the effects of friction drag are limited in comparative nature to pressure drag when considering a semi-trailer. Pressure drag is a dominant acting variable in the aerodynamic consideration of a semi-trailer. Pressure drag is caused by large pressure differentials in the wake of a trailer due to rapid flow separation creating turbulent flow characteristics. Turbulent flow characteristics can create such phenomena as a Kaman vortex street, which is a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid around blunt bodies. Such turbulent characteristics cause inefficient aerodynamic flow, due to increased pressure drag, and may even create unsafe oscillation of the trailer. In extreme cases this can result in destabilization and tip-over of the trailer and the primary vehicle.